The sources of ascending input to the medial geniculate body (MGB) of the cat were studied using the retrograde transport of horseradish peroxidase (HRP). HRP injections were made iontophoretically through micropipettes which were also used to record physiological properties at the injection sites. This technique produced small injections which appeared to be restricted to single subnuclei. The tectothalamic projection of the auditory system was found to consist of at least four distinct and separate pathways. The ventral division of the MGB receives a topographical projection from the central nucleus of the inferior colliculus (ICC) which preserves tonotopicity and provides short latency, sharply frequency-tuned responses. The medial part of the ICC projects to the deep dorsal nucleus, which contains only units tuned to high frequencies. The major inputs to the caudodorsal nucleus (DC) stem from nucleus sagulum and the pericentral nucleus of the inferior colliculus (ICP). Units in DC and the ventrolateral nucleus, which also receive input from ICP, have very broad tuning properties and late, habituating responses. Injections of HRP into the medial division (MGM) produced labeled cells scattered throughout the external nucleus of the inferior colliculus and the ventral part of ICC. This widespread input is reflected in the wide range of auditory responses found in MGM. Auditory responses in the suprageniculate nucleus were poorly defined and many units did not respond to tonal stimuli; following HRP injections no filled cells were found in the inferior colliculus, but labeled cells were found in the deeper layers of the superior colliculus and in the interstitial nucleus of the brachium of the inferior colliculus. Together with recent findings on the auditory thalamocortical projection, these results provide evidence for multiple parallel auditory pathways through the thalamus.

Parallel single unit and retrograde tracing experiments were carried out in the anesthetized cat to elucidate the representation of the auditory and somatosensory systems in the external nucleus of the inferior colliculus (ICX). Units responding to tonal stimuli were more commonly encountered in ICX and the adjacent intercollicular area (ICA) than were units with identified tactile receptive fields. Concomitantly, a larger number of retrogradely labeled cells were identified in midbrain auditory structures, following injections of horseradish peroxidase (HRP) into CRX, than were found in the combined dorsal column nuclei. Microelectrode recording revealed that the entire accessible body surface was represented in ICX, and receptive fields of individual units were usually large and mostly located on the contralateral side. HRP-labeled cells were scattered throughout the contralateral cuneate, gracile, and spinal trigeminal nuclei. tations of tone pips. Responses to complex sound were commonly observed. Binaural stimuli influenced the firing of the majority of auditory units. Labeled auditory neurons following ICX injections were found mainly in the inferior colliculus of both sides. Spread of tracer into the central and pericentral nuclei was associated with labeling of many neurons in hind brain auditory structures. Only vague suggestions of somatotopy or tonotopy were observed in the electrophysiological experiments; similarly, no topographical relationship between HRP injection site and locus of retrograde label in a given projecting nucleus could be discerned. Speculations were made, in the light of the dual convergent sensory representation in ICX, about the role of this structure in acoustico-motor mechanisms.

A series of neuroanatomical and neurophysiological experiments have been conducted within the central nucleus of the inferior colliculus (ICC) of the cat in order to determine some features of the spatial organization of the nucleus. Results from these experiments have demonstrated: (1) the origins of the auditory brain stem afferents to ICC and the topography of those projections in relation to the cochleotopic organization of ICC; (2) the segregation of at least some of those brain stem projections within ICC; and (3) ICC neurons with similar response properties (response properties which are often similar or identical to those of neurons in brain stem auditory nuclei) are often clustered within the nucleus. These results provide evidence that the laminated division of the ICC probably consists of anatomically, physiologically, and functionally distinct subdivisions and that some aspects of auditory sensation may be encoded or represented separately (i.e., in spatially distinct "regions") within the nucleus.

The representation of frequency was mapped in the primary auditory cortex (AI) of C57BL/6J (C57) mice during young adulthood (1.5-2 months) when hearing is optimal, and at 3, 6, and 12 months of age, a period during which progressive, high frequency, sensorineural hearing loss occurs in this strain. Maps were also obtained from CBA/CaJ mice which retain good hearing as they age. In AI of young adult C57 mice and CBA mice, characteristic frequencies (CFs) of multiple-unit clusters were easily identified with extracellular recordings, and a general tonotopic organization was observed from dorsal (high frequency) to ventral and caudal (low frequency). In individual cases there appeared to be deviations from the above tonotopic organization, despite the fact that inbred mice are genetically invariant. As progressive loss of high frequency sensitivity ensued peripherally, a substantially increased representation of middle frequencies was observed in AI. There was no apparent change in the surface area of the auditory cortex despite the elimination of high frequencies, and virtually the entire auditory cortex became devoted to the middle frequencies (especially 10-13 kHz) for which sensitivity remained high. Similar age-related changes were not observed in normal-hearing CBA mice. These findings indicate that plasticity in the representation of frequency in AI is associated with high frequency hearing loss in C57 mice.

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